That would be only true IF the person who voted third Party would not have voted for the other Parties under any circumstances.
If they would have voted for one of the other two Parties as a second choice, the 3rd Party has EFFECTIVELY elected the other candidate.
Your scenario only works if the 3rd Party voter would not have voted for either Party if the 3rd Party did not exist.
For a statement to be true is has to be true in all cases. If it is false in any case it cannot be true.
If one does not vote then there is no effect on the totals....if one would not have ever voted for either candidate, then there is no effect on the totals...
You are trying to say that if one does not vote for McCain then one has 'in effect' voted for Obama....and since there is no change in Obama's totals there is no effect!
Folks like you accept this 'in effect' BS idea without challenge...you need to think about things a bit more before you parrot them.